Titration Part I: Qualitative Analysis

 
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A titration reaction is a quantitative study of an acid-base reaction or a neutralization reaction. One solution of known concentration (either the acid or base) is used to determine the concentration of the other solution through a carefully monitored reaction. Typically, the titration reaction is monitored either using a pH probe or meter (which you will use today) or using a colored acid-base indicator.

Acid-base neutralization reactions can generally be described as an acid reacting with a base to yield a salt and water as shown below:

Most commonly, the H+ from the acid reacts with the OH- from the base to produce the water and the remaining ions (anion or A- from the acid and the cation or M+ from the base) combine to form the salt. (Note that in acid-base reactions, the focus tends to be on the Brønsted-Lowry definition of acids and bases.)

Acid Strength:

One of the simplest ways to classify acids is based on strength. Acids can be classified as either strong or weak. An acid (HA) that is considered a strong acid, will dissociate 100% in water, meaning there will only be H+ (or H3O+) and A- in solution and none of the original form of the acid (HA).

Any acid that dissociates less than 100% in water is considered a weak acid. Meaning if you were to look on the atomic level at a weak acid solution, you would see some H+ (or H3O+) and A-, but mostly HA.

The relative strength of acids can be found by comparing their Ka values where Ka is called the acid dissociation constant and is a measure of how much an acid dissociates in water. The more an acid dissociates in water, the stronger the acid, and the larger the Ka value.

Monoprotic or Polyprotic Acids:

Another classification of acids involves the number of H+ ions an acid can donate. A monoprotic acid will dissociate and donate one mole of H+ for every one mole of acid molecules (e.g., HCl or HNO2). A diprotic acid will produce two moles of H+ (e.g. H2SO4), a triprotic acid three moles of H+ (e.g. H3PO4), etc.

Titration Curves:

An acid-base titration curve (a plot of pH of the acid (or base) vs. volume of base (or acid) added to the solution), can give us some information about the relative strength of the acid and whether or not the acid is monoprotic or polyprotic.

In today's experiment, you will be slowly adding base to an acidic solution and monitoring the pH of the solution as the base is added. Therefore, you will most likely see a titration curve like the following:

Key features of the titration curve:

  • Low initial pH - Since the solution only contains acid in the beginning, the pH will be on the acidic side (less than 7). The initial pH of the titration depends on BOTH the relative strength of the acid and the concentration of the acid and therefore should not be used to determine the relative strength of the acid.
  • High final pH - Since excess base has been added to the solution, the pH will be on the basic side (greater than 7), but it should level off somewhat at this point in the titration..
  • Equivalence point - midpoint of the steep vertical portion of the titration curve. The equivalence point is also an indicator of relative strength of the acid. If the titration involves a strong acid with a strong base, the pH = 7 at the equivalence point. If the titration is of a weak acid titrated with strong base, the pH > 7 at the equivalence point. The exact pH of the titration at the equivalence point depends on the salt produced from the neutralization reaction.
  • Halfway to the equivalence point - At half of the volume of base needed to reach the equivalence point, there is a special data point. At this point, the pH = pKa for the acid being titrated. Ka is related to pKa by the relationship: pKa = - log Ka.
  • Titration curve shape - The characteristic S-shape of a titration curve corresponds to the reaction of one mole of protons from the acid reacting with one mole of base. If the acid is polyprotic, the titration curve will contain more than one S-curve (two for diprotic, three for triprotic, etc.). As a result, a polyprotic acid will have more than one equivalence point and pKa value.

Shown below is a sample titration curve for a diprotic acid. Note the two equivalence points and two pKa values.

 

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